Controlling bleaching operations



March 9,1926. 1,516,018

R. B. WOLF CONTROLLING BLEACHING OPERATIONSv Filed Jul'y 28, 1925 INVENTOR Patented Mar. 9, 1926.

UNITED STATESl ROBERT B. WOLF, OF NEW YORK, N. Y.

CONTROLLING BLEACHING OPERATIONS.

Application led July 28, 1925.

To all whom t may concern:

Be it known that I, ROBERT WOLF, a citizen of the United States, residing at New York city, in the county of New York, State of N ew York, have invented certain new and useful Improvements in Controlling Bleaching Operations; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same.

This invention relates to the bleaching of pulp for paper stock, and particularly to an apparatus and method permitting the more accurate control of the bleaching operation. The invention is adapted especially for use in connection with the bleaching of relatively thick stock, that is to say, pulp containing preferably more than 10% of libre and constituting a relatively non-fluid mass. In U. S. Letters Patent No. 1,433,865 I have described `a method of bleaching pulp at such relatively high concentration. The

` method can be conducted in various types lso ot apparatus, both horizontal and vertical. It depends .upon the circulation of therelatively non-tluid mass of pulp and bleach liquor. Owing to the high concentration ci the pulp the proportion of bleach inthe bleach liquor may be considerably greater than is usual in ordinary `bleaching operations and the time requiredv to accomplish the satisfactory bleaching of the material is considerably accelerated. It is essential,

however, that'the temperature of the .mass

be maintained at a relatively low pointl to avoid over-bleaching, the temperature being preferably in the neighborhood of 80 I". and

not exceeding 90 F.

In conducting bleaching operations it is possible usually to maintain the required* low temperature. Conditions arise, however, in certain cases in which the temperature of the materials entering the bleacher is already so high that l' the heat generated by the bleaching reaction is not eliminated with sufficient rapidity. Consequently there is a tendency for the temperature of the mass to rise above the maximum indicated. This rise in temperature accelerates the bleaching action so that a portion of the fibre lbecomes overbleached and is thereby weakened.

It is the object ofthe present invention to provide 4for the satisfactory control. of the bleaching operation so that overbleaching is avoided. This is accomplished by the re- Seral No. 46,533.

moval of the surplus heat as rapidly as it is generated so that the temperature of the mass is maintained constantly atsubstantially the desired point.

I have discovered that the essential control can be accomplished by the introduction of air to the bleacher in s uiicient volume and particularly with suiticient velocity to ensure the removal of the heat. A certain proportion of the heat can be removed as sensible heat, that is to say, by transfer to the air at a lower' temperature. To increase the heat transfer the air may be cooled before it is introduced to the bleacher. ticable always to cool the air and its temperature is often a/fery close to that of, the mass undergoing bleaching. It is possible, nevertheless, to remove the heat as latent heat, and in the'preferred mode of operation the major portion ot' the heat which is withdrawn from the mass Ais carried oit as latent heat by evaporation of a portion of the moisture content by the entering air. This evaporation is accelerated by introducing the air under a suflicient velocity and under conditions which cause it to permeate the pulp. The cooling ot' the pulp is thus accomplished by a combination of the Withdrawal of heat as sensible and as latent heat, maximum advantage being "taken of both of these metliods ot cooling. As the result of the application of air as indicated to the pulp, the

It is not praci tendency to overbleach is avoided. Notwithstanding this reduction in temperature, I

have found that the bleaching action is` that the time required to finish the bleaching of a given quantity of pulp is reduced Without danger ot wea-kening the fibre.

By directing the air so that it impinges against the tibi-es of a continuously moving surface layer lof the pulp, it rebounds from the surface and carries away a considerable Aportion of the productsv of decomposition direct-ly after they are formed during the bleaching operation. f

In the preferred method of conducting the bleaching operation a vertical type bleacher is used, the bleacher being provided with uneans to cause a circulation of the concen-,

trated and substantially nontlnid pulp. This circulation may be accomplished by the "provision of n a vertical tube within the bleacher. enclosing a screw conveyer which lifts the pulp and discharges it at the top of the tube. From the top of the tube the pulp is distributed by an lapron provided for that purpose and descends therefrom byk gravity until it is again. picked up by the conveyer. In such an apparatus lthe pulp which has been previously dewatered to a concentration of approximately to 4.0%, is mixed With a suitable proportion of the bleaching agent which may contain, for example, from -20 to 30 grams of available chlorine per liter.

The amount of bleach in the bleaching liquor so added will vary with the character of the pulp. The amount of bleaching liquor to be added Will depend upon the concentration of the pulp and should be such as to produce a pulp mass containing preferably more V"than approximately 12% of pulp. The

bleach is most effective when the pulp mass has a concentration of from 18 to 25% al-` though it is possible to. treat the pulp at even higher concentrations. y

The mixture of pulp and bleaching liquor is caused to circulate in the bleacher by being lifted continually and discharged into the space surrounding the conveyer. During this operation the temperature of the mass should be maintained preferably at `about to 85 F., and in any event at not materially exceeding F. So long as the.

bleaching liquor and pulp enter the bleach at a suiiiciently 10W temperature no dit* ticulty will be experienced in avoiding high temperatures. It, however, the bleachinr liquor and pulp enter at temperatures o -i'rom 85 to 90 F. the temperatureof the pulp mass will tend to rise and it may reach a temperature of F. or higher.' Suchl vto convey the required volume of air With nozzles directed `upon the pulp as it iiovvs over the'apron at the top of the conveyer. The entire exposed surface of pulp from thc Iconveyor tube to the side of the tankcon- 'stitutes a relatively thin layer of fibrous lumps. Thissurtace layer is continuously being renewed as fthe additional pulp is discharged.I from the tube.- The newly discharged pulp spreadsout, as a new surface layer, while that portion of the pulp formerly constituting layers is v'permitted to descend in circulation. The amount of air is .calculatedwith reference to its temperature and relative vhumidity asl it enters the bleacher and the required drop in temper atureV therein. Thus in a bleacher of. 4 tons capacity `with the, materials centering at about 85 F., it is desirable to introduce` air ata velocity of approximately 4000 feet per minute through about thirty nozzles having a diameter of approximately 3' inches at 70 their outward ends. This requires an air pipe of about 16 inches in diameter and a source of air under a sufficient pressure to maintain the delivery of air to the pulp at the indicated velocity. If tlie-air enters the bleacher at a relative humidity ofapproximately 50%, it Will'be discharged with a humidity of about 80%, indicating thus the evaporation of moisture in the pulp. This I evaporation of the moisture is accompanied by a drop in temperature of approximately 15F. and the pulp is vthus maintained at a temperature suiliciently low tol avoid the eiect `of overbleaching. The amount of heat to be withdrawn in any case will depend, of' course, upon the particular conditions existing inthe bleacher and these' in turn are aiiiected by the temperatures` of the materials entering the bleacher. In conducting the operation, therefore, itis necessary to consider the conditions in each specific case and to. so regulate the flow of air to the bleacher as to ensure the desired reduction ot' tei'nperature therein and the maintenance of;` the pulp at a substantially uniform temperature which is adapted lto permit the most effective bleaching Without loss of fibre strengtl-i. The air entering thel bleacher must` ot course, be Withdrawn and for that purpose it is sufiicient to provide an outlet pipe of suitable size, for example, approxi` mately 29. vinches in diameter so that it can pass freely therethrough to the atmosphere.

`Since'the maior portion of the heat is ioo Withdrawn by the evaporation of moisture and the conversion of heat into latent heatml of vapo-rization, it is essential to the efficient operationof the methodthat the air be directed upon the pulp at a sutlicientvelocity and under such conditions that the surface lumps of pulp are blown apart and the air is permitted to circulate in Contact Withthe fibers thereof. The contact of the air with the fibers permits maximum evaporationl and of moisture from the pulp, and -a consequent maximum reduction in temperature. AThis is accomplished most satisfactorily in the apparatus hereinafterl described embodying features which have been mentioned.` In this apparatus the air nozzles are directed upon the apioii in close proximity thereto ensures the utmost etliciency in the removal and as the pulp flows over the apron itis thoroughly 'aerate,d, all of the fibre being subjected more, or less'to contact with the y streams of air impinging thereon.

T lie enormous evaporating and cooling area presented by the iibers vWhich protrude from ,the surface lumps of pulp'enable the impinging streams 'ot `air to not only absorb moisture with great rapidity, but also eration.

Gil

to sweep ofi' from the fiber surface considerable of the volatile decomposition products which are formed during the bleaching op- It is the presence of these decomposition products which slows do-wn the bleaching action, particularly at the last end of the period when the bleaching agent is nearly exhausted, and I have discovered th-at their removal by the impingement of a large quantity of rapidly moving air so hastens the bleaching act-ion that it is entirely unnecessary to apply heat to obtain a vbright colored pulp.

T he accompanying drawing is a crosssec tional view of one form of apparatus adapted for the practiceof the invention. Other types of apparatus may be employed, the invention being applicable in the manner hereinbefore indicated wherever it is essential to control bleaching, andparticularly to reduce the temperature to which the pulp tends to rise during the bleaching oper-ationn Referring to the drawing, indicates a vertical tank preferably constructed of concrete or other suitable material adapted to withstand the action of the ble-aching liquor. A tube 6 is supported within the tank and encloses a screw conve-yer Z which is supported on a shaft 8 mounted in suitable bearings on a cover 9 of the bleacher. The shaft is adapted to be driven through -a bevelgear 10 and bevel pinion 11, the latter being supported on a shaft 12 having a pulley 13 or other means of transmitting power. A

scraper 14 is disposed at the bottom of the tank 5 to draw the material into the tube 6 and thus to permit lifting thereof through the tube to the upper end thereof. A n apron 15 is supported at the upper end of the tube and the pulp is discharged thereon and flowstherefrom into the mass which is maintained in the bleacher and-which circulates constantly under the action of the conveyer.

A circular pipe 17 is disposed in the top of the bleacher and isad-apted to'be supplied with air under suitable ressure through an inlet l18. A plurality o nozzles 19 are directed from the pipe 17 toward the apron 15. These .nozzles are preferably of slightly smaller diameter at their outlets so as to increase the velocity of the air flowing therefrom. A discharge pipe y 20 passes through the top 9 of the nleacher to deliver the air after ithas absorbed moisture to the atmosphere. A discharge -Valve 21 is also `provided ,at the bottom of the bleacher for the removal of the pulp afterA it has been s uflicient-ly bleached@ 1 In the apparatus as described the constantly circulating pulp containing the bleaching liquor is discharged over the apron.

the chlorine and the temperature of the mass tendsto rise. If the temperature 1s During its passage thrbugh the" bleacher heat is generated by the action of already somewhat high the increase in temperature will cause a too violent bleaching action and consequent weakening of the fibre. This effect is avoided, however, by the actionof the air flowing fro'm the nozzles and impinging upon the pulp as it passes over the apron. The air brushes out the fibre.. on the apron and in contacting therewith evaporates a portion of the moisture content, the water vapor being absorbed by the air and passing therewith through the discharge pipe. The evaporation of the moisture absorbs heat from the mass and the temperature is reduced materially.- -If the air entering the bleacher is lower in temperature than the pulp in the bleacher, it will absorb heat in its passage there-through causinga reduction in temperature of the pulp mass. This reduction,

due to withdrawal of sensible (heat, can bte/ augmented if desired by cooling the entering air. 'The most effective coling is, however, accomplished by the withdrawal of heat as latent heat of vapor-ization and this is controlled by the velocity of the air as it impinges upon the pulp. By increasing or decreasing thc velocity of the air the amount of cooling can be varied to meet .therequired condition and to maintain a substantially constant temperature within the bleacher. This ensures the maximumA effectiveness of the bleaching operation.

It. is customary at the time when the bleacheris being filled with pulp, to'add the bleach liquor, and during this filling period loo this mixture of pulp and bleach liquor vpasses over the apron, as a result of which it falls olf' the edge of the apron into the main body of the pulp below. As the mixture showers off of the apronfin fibrous fIS- lumps, the air from the nozzles strikes these lumps, causing rapid evaporation. The velocity of the air being Iapproximately 4,000 feet per minute, the entering air, bec au'se of the nozzle effect, is carried down 110 with the pulp and`finally iescapes from. the bleacher through lthe vent pipe, after having absorbed both moisture and volatile decomposition products.

" It should be noted that as a large part ofv the bleaching action takes place immediately after the bleaching agentcis added to the pulp, it is there-fore quite advantageous to -impinge air upon the pulp as it showers fromI the apron of the.v conveyor tube. In this way, most of the heat of chemical combination generated during the filling period is removed. As `pointed out in my previous patent referred to above, Lit

is especially desirable lto keep down the 15-5- the spaces between the lumps of pulp, and because of the non-fluid character of the 'pulp mass this air cannot escape; conse- -quentlyit. is carried downward due to the movement of the pulp where it finally comes under the influence of the scraper. These entrapped bubbles of air being under pressu're because of the weight of the overlying Vmass of pulp, are brought into intimate con- 1 products, are continuously being carried up the conveyor' tube and discharged from the top thereof, after which they escape through the Vent pipe. Y

Various changes may be made in the details of operation including the method of applying the air to the pulp as well as in the Vapparatus employed for thel purpose without departing from the invention or sacrificing any of the advantages thereof.

I claim:

l. The.` method of controlling the bleaching of paper pulp, which comprises subjecting the mixture of pulp and bleaching agent to the action of cooled air.

2. The method of controlling the bleaching of paper pulp, which comprises causing streams of air substantially free from the' gaseous decomposition products of the .bleaching reaction to impinge upon the mixture of pulp and bleaching agent.

- 3. The method of controlling the bleaching of paper pulp, which-comprises causing the mixturey of pulp andl bleaching agent to pass in a relatively thin layer in contact lwith a directed current of air substantially 'free from the gaseous decomposition products of the bleaching reaction. A

4. The method of controlling thebleaching of paperpulp, which comprises causing the mixture of pulp and bleaching agent to L f pass in a relativelythinnlayer in contact with air substantially free from the gaseous decomposition products of the 'bleaching reaction directed thereon in streams. f

5. The method of controlling the bleaching of paper'pulp, which comprises directing streams of' air upon -a flowing layer of the mixture of pulp and bleaching agent, the velocity of the air being sufficient to y cause substantialevaporation of the mois-` ture content of the pulp.

6. The method of controlling the bleaching -of paper pulp, which comprises reduc-l ing vthe temperatureof the mixture of pulp and bleaching agent by causing a thin layer ing of paper pulp, which comprises reducand bleaching agent by transfer of heat to air substantially free from the gaseous decomposition products of the bleaching reaction directed thereon as latent heat of Vaporization.

8. The method of bleaching which Vcomprises directing streams of air upon a relatively non-fluid mass of a mixture of pulp and bleaching agent containing not less than ten percent pulp for reducing the temperature of the mixture.

9. The method of controlling the bleaching of paper pulp, which comprises reducingl the temperature of the mixture of pulp and bleaching agent by the action of streams of cooled air impinging thereon.

l0. The method of controlling the bleaching of paper pulp, which comprises reducing the temperature of the mixture of pulp and bleaching agent by transfer of heat to cooled air in contact therewith.

l1. The method of controlling the bleaching of paper pulp, which comprises reducing the temperature of the mixture of pulp and bleaching agent by transfer of heat as latent heat of evaporation from relatively thin layers thereof to air substantially free from the gaseous decomposition products of the bleaching reaction inA contact therewith.

12:1.The method of controlling the bleaching of paper pulp, which comprises'reducing the `temperature of the mixture of pulp and bleaching agent by transfer of heat as latent heat of Vaporization to streams of air substantially free lfrom the gaseous decomposition products of the bleaching reaction impingingthereon.

13. The method of controlling the bleaching of paper pulp, which comprises reducing the temperature of the mixture of pulp and bleaching agent by transfer of heat as latent heat of vaporization to streams of air substantially free from, the gaseous decomposition products of the bleaching reaction impiiging upon a relatively 'thin layer ofthe pu l The method of controlling the bleaching of paper pulp', which comprises'eirculating the mixture of pulp and bleaching agent andcausing a relatively thin layer of the mixture to be vmoved into a current of thereof to be subjected to a controlled air-dof the mixture to be moved into the path of current substantially free from the gaseous decomposition products of the bleaching reaction, for transferring heat to theair.

'a current of airsubstantially free from the A gaseous decomposition products of the bleaching reaction, which im inges thereon. 65 7. The method of controlling the bleachm16. The method of contrglling the bleach- 130 ing of paper pulp, which comprises circulat-- ing a relatively nonfluid mass of pulp and bleaching agent containing not less than ten percent. of pulp. to a position in which it is subjected to a controlled air currtnt.

1T. The method of controlling,r the bleaching of paper pulp. which comprises circulating a relatively nonfluid mass of pulp and bleaching agent containing not less than ten percent of pulp. to a position in which a coiitrolled air current inipinges thereon.

1S. The method of conti olling the bleaching of. paper pulp, which comprises circulating a relatively nonfluidoniass of pulp and bleaching agent containing not less than ten percent of pulp, to aposition in which a controlled air current impinges upon a relatively thin layer thereo 19. The method of controlling the bleachof paper pulp. which comprises circulating the mixture of pulp and bleachingagent by lifting it, permitting it to descend by gravit v, and subjecting it to a current of air at the stage ofthe cycle between its highest point and the ascend.

20. The method of controlling the bleaching of paper pulp, which-'comprises circulating the mixture of pulp and bleaching agent by lifting it, permitting it to descend by gravity, andsubjecting it to air impinged thereon in streams between its highest point and the point where it begins to ascend.

21. The. method of controlling the bleaching of paper pulp, which comprises circulatingl themixture of pulp and bleaching agent by liftiii it, permitting it to descend by gravity, an subjecting a relatively thin layer thereof to directed air currents at the stage of the cycle between its highest point and the point where it begins to ascend.

20. The method of bleaching, which coinprises introducing air to a descending relpoint where itl begins to atively non-:fluid mass of a mixture of the pulp and bleaching agent containing not less than ten percent pulp whereby the air is trapped and carried downward with the pulp.,

:23. The method of bleaching` which coniprises introducing air to a descending ielatively non-Huid mass ofa mixture of the 'pulp and bleaching agent containing not less than ten percent of. pulp whereby thel air is trapped and carried downward with the pulp, and subjecting the mass containing entrapped aii to agitation.

24. The method of bleaching, which comprises introducing air tol a descending relatively non-fluid mass of a mixture of the pulp and bleaching agent. containing not less than ten per cent of pulp whereby the air is trapped and v,carried downward with the pulp, and subjecting the mass containing entrapped air to agitation while the air is under compression. v v

25. In an apparatus for bleaching paper pulp, the combination with a. bleaching chamber, of means to elevate the pulp within the chamber, Kmeans through ywhich the portion constituting the upper surface is permitted to assume a lower level, and means for directing a gas upon the. surfacet of the pulp at a time during its passage from the upper to the lower level.

2G. In an apparatus for bleaching paper pulp, the combination of bleaching chamber, a substantially vertically arranged conduit through which the pulp is adapted to be `elevated and overflow therefrom, an elevator within the yconduit for Acausing an' upward movement ofthe pulp/therein, and means for directing a gas upon the pulp as it overflows from the upper end of theconduit.

2T. In an apparatus for bleaching paper pulp, the combination of a bleaching chamber,

an elevator therein to lift themixture of pulp and bleaching agent, an apron at the top of the elevator over which the pulp is discharged and means for directing gas upon the pulp as itfpasses over the apron.

,. Q8. In an apparatus for bleaching paper Cpulp, the combination of a bleaching chamber, an elevator therein to lift the mixture of pulp'and bleaching agent, an apron'at the top of the elevator over which the pulp is discharged and means for directing streams of gas upon the pulp as it passes over the apron.

29. In an apparatus for bleaching paper pulp, the combination of a bleaching chamber, an elevator therein to lift the mixture of pulp and bleaching agent, an apron at the top of the elevator ove;` which the pulp is discharged, a pipe in the bleaching chamber and nozzles connected thereto to direct streams of gas upon the pulp as it passes over the apron. X j 'In testimony whereof I atlix 1my signature.

ROBERT B. WOLF. 

